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Transcript
J. Pulickeel SPH4U1 February 2010 Newton’s Three Laws 1. An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless acted upon by an external force 2. 3. For every action, there is an equal and opposite reaction. Equal in magnitude and opposite in direction. Explain what’s Happening using Newton’s Laws…. The ladder undergoes the same accelerated and decelerated motion that the truck experiences. Once the strap is no longer present, the ladder is more likely to maintain its state of motion. If the truck were to abruptly stop the ladder in motion would continue in motion. Assuming a negligible amount of friction between the truck and the ladder, the ladder would slide off the top of the truck and be hurled into the air. Once it leaves the roof of the truck, it becomes a projectile and continues in projectile-like motion. In the picture below, Kent Budgett is pulling upon a rope which is attached to a wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 N. Kent is pulling ... a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. the same force in each case. Kent is pulling with 500 N of force in each case. The rope transmits the force from Kent to the wall (or to the elephant) and vice versa. Since the force of Kent pulling on the wall and the wall pulling on Kent are action-reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push and pull. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it. 3. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet. The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional. Free Body Diagrams List all the forces acting on the hockey puck The Normal Force: The force acting perpendicular to the surface of contact The Force of Gravity: The force pulling an object towards the centre of the earth Force of Friction: the force resisting the motion of solid surfaces. - Static Friction: friction between two solid objects that are not moving relative to each other - Kinetic Friction: occurs when two objects are moving relative to each other and rub together. Fs S FN Fk K FN The Applied Force: The force applied. Calculating the Forces... The Normal Force IN THIS CASE, the normal force has the same magnitude as the Force of Gravity. The Coefficient of Friction (µ) The coefficient of Friction a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations Fs S FN Fk K FN This value will always be given to you! FBD: Level Planes vs. Ramps The block is not moving forward because there is no horizontal applied force. What would the FBD look like if a force was applied, but the block was not moving? What would the FBD look like if a force was applied, but the block was moving? FBD: Tension What would the FBD for the following setup look like? + y FB FA + x FC 9.8N Is the object in motion or at rest? = = = = = = = FBD: Tension The sign below hangs outside the physics classroom, advertising the most important truth to be found inside. The sign is supported by a diagonal cable and a rigid horizontal bar. If the sign has a mass of 50 kg, then determine the tension in the diagonal cable which supports its weight. Since the mass is 50 kg, the weight is 490 N. Since there is only one "upward-pulling" cable, it must supply all the upward force. This cable pulls upwards with approximately 490 N of force. Thus, sin(30°) = (490 N ) / (FT). The tension is 980 N. FT Fg FBD: Level Planes vs. Ramps An object placed on a inclined surface will often slide down the surface. The rate at which the object slides down the surface is dependent upon how inclined the surface is; the greater the incline, the faster the rate at which the object will slide down it. You will note that in both cases, the box not only fell down, but it also travelled horizontally. That means there is an x and y Force acting on the box! FBD: Level Planes vs. Ramps + y In this situation, no external force is being applied, but the block slid down the ramp. +x This is because some of the force of gravity is pulling the block into the plank of wood (opposite to the Normal Force) Some of the Force of Gravity is overcoming friction to cause the block to move forward . This means that Fg is the sum of FN and Fx PROBLEM 1: A board is inclined at 35°. The forces acting on the block are illustrated. Find the net external force on the block and determine whether it will move. 11 N 18 N + y y +x 11 N 18 N 35° 22 N Fg,y 35° 35° 22 N Fg, x x PROBLEM 1: A board is inclined at 35°. The forces acting on the block are illustrated. Find the net external force on the block and determine whether it will move. + y y +x 11 N 18 N 35° 13 N 18 N 35° 22 N x 13 N Therefore the net force is 2 N in the positive x direction.